The field of the invention is telecommunication cables having cylindrical spacers having open exterior grooves for the insertion therein of longitudinally extending telecommunications members such as insulated copper wires or coated light waveguides or ribbons containing light waveguides. Such cables are sometimes referred to as slotted core cables. The invention concerns apparatus and methods for the manufacture of the grooved spacer, sometimes called the slotted core.
It is well known that such grooved spacers may comprise a layer of plastic extruded over a core strength member. One or more grooves are formed in the external surface of the plastic, and a telecommunications member is inserted into a groove. The grooves may follow paths which are helical in shape or which have a direction of lay which reverses at periodic intervals.
With the more recent trend toward inserting a stack of light waveguide ribbons in a groove, especially in cables having a high fiber count and high fiber density, irregularities in the exact shape of the floor and walls of U-shaped grooves become more significant in the performance of the cable. The portions of the plastic material between adjacent grooves and forming the groove walls are referred to as the ribs of the spacer. Even if the grooves are initially extruded having the desired U-shape, a wall of a groove may slump inward into the groove due to the force of gravity, the angular momentum on the legs caused by rotating the spacer during manufacturing, or flow in the extruded melt due to cooling effects. Spacers of high density cables become more susceptible to slumping as the width of the grooves increases to accomodate wider ribbons having more light waveguides therein, because the width of the plastic at the base of a rib between the corners at the bottom of adjacent grooves becomes smaller. The spacers also become more susceptible to slumping as the height of the grooves increases. Such wall slumping can cause unacceptable attenuation in light waveguide ribbons inserted into the groove, especially when the cable is bent.
Hulin, U.S. Pat. No. 4,272,472, describes a prior art technique for making a cylindrical member having near its exterior surface a plurality of rounded ducts, each of which is almost closed at the outer surface of the cylindrical member by a pair of oppositely facing lips directed towards each other. Longitudinally extending pins mounted to a crosshead extruder tip and distributed along a circle of diameter smaller than the die opening extend downstream of the die to completely fill and delimit the ducts in the extruded member. A flexible wire having a smaller diameter than the pins is mounted at the distal end of each pin. To obtain the ducts following helical paths, the pins and the tip are rotated by a motor. A knife or wire at the distal end of each flexible wire is used to cut a narrow passage to the exterior surface of the cylindrical member between the lips of the duct. Specialized extrusion equipment is required to rotate the tip.
Yataki, U.S. Pat. No. 4,474,426, addresses the problem of exterior groove dimensional stability in a grooved spacer for telecommunications cable by grinding the grooves following extrusion of the cylindrical member. However, such grinding results in wasted plastic, introduces an extra processing step, and requires specialized equipment.
Matsuno et al., U.S. Pat. No. 4,814,133 addresses the problem of exterior groove stability in a grooved spacer for telecommunications cable by using a two-step extrusion of the plastic, to provide a central strength member, an intermediate annular plastic layer over the central strength member, and an exterior plastic layer having grooves in its exterior surface. The examples given include grooves having a depth of up to 2.4 mm in high density polyethylene (HDPE); however, groove depths of up to 1.4 mm are shown when linear low density polyethylene is used. The method reduces the volume of the plastic in the grooved exterior layer, but does not address postextrusion groove dimensional stability.
To address the problem of groove distortion in a grooved spacer for telecommunications cable when the extruded plastic spacer contacts cooling water in a cooling vat, Schneider, U.S. Pat. No. 5,380,472, provides a plurality of shaping disks housed in the cooling vat. The shaping disks have web-like projections extending radially inward toward the axis of the grooved spacer. Each projection is inserted into a groove. The shaping disks may be used in making a grooved spacer formed of HDPE having six grooves, each having a groove width of 1.5 mm and a height of 2.7 mm made using the shaping disks.
In practice, the risk of the grooved spacer becoming lodged in the shaping disks during processing is significant. The extrusion of a spacer formed of a plastic material having a melt flow index higher than that of HDPE and having grooves of a depth greater than 3.0 mm is difficult using the shaping disks. The disks must be mounted to each other by rods or the like to maintain exact spacing and helical alignment of the projections. The resulting apparatus is expensive to build and can be difficult to properly clean during a back-up of the plastic material. Other disadvantages are that the distance between the extruder die and the cooling vat cannot be adjusted after the apparatus is mounted in the cooling vat, and the apparatus is relatively intolerant to variations in the longitudinal or angular velocities of the spacer.